Many different mass analyzers are used in the field of mass spectrometry. The most common mass analyzers include ion-cyclotron resonance (ICR) or Fourier transform mass spectrometer (“FTMS”), ion traps, quadrupole, time-of-flight, orbitraps and magnetic sector instruments. All of the commercial versions of these instruments detect the ions by using an ionizing particle detector or by measuring an electrical image current as done when using the Fourier transform technique for FTMS and the orbitrap. Although there has been experimental work using photon emission to make ion measurements inside an ion trap, no known commercial instrument with any of the above mentioned mass analyzer types use optical detection of ions inside the mass analyzer to determine the m/z and/or the presence of select chromophores. In the pioneering work of, Wuerker, Shelton and Langmuir in 1959, they used a 3D-quadrupole field ion trap or what is often now called a “Paul” trap to teach how charged aluminum particles could be confined and photographed in dynamic equilibrium. They applied a voltage across the end caps of the ion trap to measure the resulting frequency of the charged particles at resonance and recorded interesting tell tale photographs of the charged particle trajectories. Holes were machined in the ion trap electrodes to make these observations and when the particles reached a stable orbit they noted the voltage and frequencies applied to the electrodes. They used a conventional 35 mm microscope camera and exposed the film for 0.1-0.2 seconds. They did not purposely move the ions to specific locations in the trap to image or photon count or count from specific regions of the ion trapping volume, but instead they scanned the RF or resonant frequency for mass analysis and recorded for the first time, the complete motion of the trapped charged particles in the ion trapping chamber. Others have since followed Wuerker, Shelton and Langmuir's work and have used improved light sources, such as lasers; detectors, such as photo multiplier tubes; different analyzers and optical techniques such as laser induced fluorescence (LIF) to make improved measurements. In 1975, LIF of a non trapped molecular ion was first reported for N2+ by Engelking and Smith. In 1977, Miller and Bondybey followed by showing the LIF of non-trapped CO+ and CO2+ and later with Sears, in 1978-81, of various fluorinated benzene cations. Between 1980-82 CD+ and BrCN+ and N2+ were trapped for the LIF measurements using a Paul trap by Grieman, Mahan and O'Keefe. There have also been reports of LIF in Penning ion traps by Drullinger and others. Welling, Thompson and Walther in 1996 [IJMSIP, 172, p 95.] used a linear ion trap (LIT) to determine the absolute gas phase photo dissociation cross-section of the ionic molecular complexes. They made complexes of MgC60+ by in situ collisions inside the LIT and then dissociated them using a laser shot off-axis. The resultant signal was recorded using mass spectrometry or by using laser induced fluorescence (LIF) of the Mg+ product ion. Welling, Schuessles, Thompson and Walther followed those experiments in 1998 to study ion molecule reactions in the gas phase and again used optical spectroscopy on the trapped ions in a LIT. They used parametric excitation of the secular motion to generate the mass spectrum with ejection amplitudes of 1 Vpp and scan rates of 200 kHzs−1. They calculated that they trapped 106 ions in a 10 cm long trapping volume which facilitated the spectroscopy measurements even with an off-axis laser and detector. Co-linear excitation was not done. They measured total cross-sections of MgC60+ by LIF and photo-dissociation. Photons from the strong LIF transition from Mg+ were detected perpendicular to the ion axis in a pump probe approach. In these studies they did not do mass analysis using LIF. In 2001, Wang, Hendrickson and Marshall reported on LIF optical measurements of hexafluorobenzene from inside an ion cyclotron resonance mass spectrometer, but they did not use LIF for mass analysis.
Nakamura and coworkers [JAP 2001, 89, 2922], measured various resonances in a linear combined trap in 2001 and measured these frequencies by using a fast Fourier transform method.
In 2001, Schlemmer, Illemann, Wellert and Gerlich [JAP, 90, p 5410] demonstrated mass spectrometry of 500 μm SiO2 particles in a Paul-like trap using a light scattering nondestructive method. Their ion trap used end caps, but the field in the r-dimension was generated by eight rods to allow for good penetration of the laser beam into the trap and thus they did not incorporate a ring electrode. In this experiment the particles were illuminated inhomogeneously so that the scattered light would be modulated at the secular frequency of the charged particles trajectories thus allowing the calculation of the m/z using a fast Fourier transform. The laser beam density was used to achieve this measurement and a photodiode detector was used to detect the photon emissions.
In 2002, Khoury, Rodriguez-Cruz and Parks used electrospray ionization (ESI) and a Paul trap to make pulsed fluorescence measurements of trapped molecular ions for rhodamine 640 and Alexa Fluor 350. For ion trapping during ion injection, the helium was pulsed to 2×10−4 Torr for 0.25 to 0.5 seconds. Their calculations showed that for 103 trapped ions, approximately 1% of these were detected by photon emissions through a hole in the ring electrode. The low percentage of ions detected per pulse was partially due to a maximum laser beam overlap of 3-15% of the ion cloud and the detection of the photons emitted through a 1.2 mm hole to the photomultiplier. They were able to eliminate background laser scattering during the pulse by delaying the measurement which greatly improved the signal-to-noise level. Measurements were taken over minute exposure times. They did not use the LIF signal to determine m/z. In 2003, Danell and Parks showed that fluorescence resonance energy transfers (FRET) measurement can be made on oligonucleotides duplexes in an ion trap using spectroscopy and a 3D-quadrupole field ion trap. They stressed that the elimination of background scattered radiation at the detector was essential to make their measurements.
Baba and Wald studied the sympathetic cooling rate of gas-phase ions in linear ion traps in 2002. In their experiment a laser beam was directed on-axis in the LIT. Marshal et al. determined fluorescence lifetimes of ions in a Penning trap and demonstrated the feasibility of resolving these ions from a heterogeneous mixture.
In 2003, Cai, Peng and Chang used a dual Paul trap to do LIF on labeled polystyrene particles. The first trap was used to trap all the charged particles while the second trap received select particles from the first and was used for LIF. They used a frequency scan from 0.5 Hz to 50 kHz for mass analysis. Particles up to 27 nm were analyzed by this method as well as fluorescently labeled IgG at 150 kDa. In 2005, Peng, Yang, Lin and Chang showed high precision mass determination for single polystyrene spheres using a Paul trap [Anal. Chem., 2005, 77, 7084.]. In these experiments they used a single trap and monitored the star like trajectory pattern to determine the mass following Hars and Tass [JAP 1995, 77 p 4245.].
And finally, Iavarone, Meinen, Schulze and Parks made fluorescence measurements to probe peptide conformations in their Paul ion trap and did the mass analysis using conventional methods.